Fibber,
CFS2 runways (coral, dirt, steelmat) are 16bit 444 extended bmp (see SCENEDB/RUNWAYS).
The older runways (main texture folder) seem to be old R8 bmps, limited colours & limited pixel size, & may not be that easy to modfy to get extra detail into.
I think a better idea is to do what Maskrider & Pen32win did, & that's create dedicated runway/ blended backdrop bmps for the tarmac/ gravel/ concrete surfaces at individual airfields & create invisible runways in FSSC.
Hope this helps.